The photosensitive layer in an organic photovoltaic (OPV) or an organic photodetector (OPD) device is composed of at least two components, a p-type semiconductor such as a polymer, an oligomer or a defined molecular unit as first component, and a n-type semiconductor such as a fullerene or substituted fullerene, graphene, a metal oxide, or quantum dots as second component. In recent years, the stability of OPV devices has been investigated. The interactions which take place during OPV operation are complicated which generates many pathways for OPV device degradation. One approach to impact the stability of OPV devices is to modify the n-type semiconductor. Recently the approach to using modified n-type semiconductors and n-type semiconductor mixtures has been limited to only a few selected candidates, like PCBM-C60.
More recent approaches to stabilizing OPV devices by formulation of n-type semiconductors as promising alternatives to PCBM-C60 are limited, for example, to using mixtures of monosubstituted and polysubstituted fullerenes, as disclosed for example in US 2014/0116510 A1, using mixtures of unsubstituted, monosubstituted and polysubstituted fullerenes, as disclosed for example in US 2013/0306944 A1, or using different sizes of fullerenes, as disclosed for example in C. Lindqvist et al., Appl. Phys. Lett., 2014, 104, 153301. However, the physical properties of these mixtures such as solubility, light stability, power conversion efficiency, and thermal stability limit their broad commercial application.
Thus there is still a need for fullerene materials which show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, a good processability, especially a high solubility in organic solvents, and high light and thermal stability, and are suitable for use as n-type semiconductors in OE devices, especially in OPV and OPD devices.
It was an aim of the present invention to provide fullerene materials that provide one or more of the above-mentioned advantageous properties. Another aim of the invention was to extend the pool of n-type OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing mixtures of substituted fullerenes as disclosed and claimed hereinafter.
In particular, the inventors of the present invention have found that, instead of mixing fullerenes having different size or different degree of substitution, providing mixtures of substituted fullerenes with specific variation in the nature of the substituent offer significant unexpected advantages in the stability of OPV devices over other approaches.